Wes, others: there is a thread in the LaRue forum on AR.15, that some of the owners were not happy with the fit, at the rear between upper and lower. I see the gap on your rifle, too, Wes. Esp. Pronounced on the left side. It is like there is an AR10 pattern trying to meet a DPMS pattern. On is sharp edged and the other rounded.
is this a real issue, or just optics?
I didn't notice this on my own rifle until it was pointed out in this thread. What I did observe is that the upper and lower are a perfect bank vault fit, yet the push pins will come out with just finger pressure. Maybe I was too dazzled with the attractive lettering and flawless Cerakote. When I took a close look at the area where the upper and lower join at the rear in the LaRue rifle, I can see two different contours.
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I think I know why the lower is built the way it is.
The LaRue rifle has extra material where the threaded lower receiver extension attaches to the lower receiver. This area was identified as a point of structural failure in the M16A1, and extra material was also added here in the M16A2 and M4 lower receivers. LaRue apparently went a step further.